1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display that compensates an hourglass phenomenon using a backlight unit.
2. Description of the Related Art
Among various kinds of thin displays having been developed and marketed, liquid crystal displays (LCDs) are widely used in the market due to their various advantages. LCD is a display device that includes two substrates, each having an electric field generating electrode formed on one side of the substrate, and a liquid crystal material injected into a gap defined between the substrates disposed to make the electrode-mounting sides face each other so that liquid crystal molecules can be arranged by electric field generated through application of voltage to the opposite electrodes to display images through adjustment of variable light transmittance.
Unlike other types of thin displays such as plasma display panels (PDPs), field emission displays (FEDs), etc., LCD is a non-emissive device, so it cannot be used without illumination. To solve this problem of LCD, a backlight unit is included therein as a surface light source of high brightness for the purpose of enabling operation of LCD in dark places.
In general, liquid crystal molecules have anisotropy and properties that the anisotropy of a liquid crystal cell or a film comprised of the liquid crystal molecules varies based on distribution of the liquid crystal molecules and distribution of tilt angles with respect to a substrate. Such properties of the liquid crystal molecules are major factors that change polarization of light depending on an angle of viewing the cell or the film comprised of the liquid crystals.
Recently, with an increasing demand of LCDs having high image quality and large size, a TN mode LCD adopts a wide view (WV) polarizer to solve its drawback of a narrow viewing angle.
FIG. 1 schematically shows the construction of the WV polarizer applied to conventional LCDs. Referring to FIG. 1, the WV polarizer 10 includes a polyvinyl alcoholic (PVA) film 11, which is a polarization film to control intensity of a transmitted beam based on a polarization degree of an incident beam, a tri-acetyl cellulose (TAC) film 12 and a T-SWV film 13 respectively adhered to both sides of the PVA film 11 to protect and support the PVA film 11, a compensation film 14 adhered to one side of the T-SWV film 13 for optical compensation of a liquid crystal cell, an adhesive 15 applied to an outer surface of the compensation film 14 to bond to a glass of the liquid crystal cell, and upper and lower protective films 17 respectively adhered to the adhesive 15 and the TAC film 12 to protect the WV polarizer.
As shown in FIG. 1, the WV polarizer 10 has a multi-layer structure of the films, which have different expansion and contraction coefficients for heat and humidity. So, when WV polarizer 10 is exposed to external heat and humidity for a long time period, some portions of the WV polarizer 10 become weakened. Detailed description of this will be described hereinafter.
In FIG. 1, the respective films are adhered on top of one another on the substrate by applying stress to the films in opposite diagonal directions. Specifically, if one film is adhered by applying stress in a diagonal direction, the next film is stacked on the adhered film by applying stress in an opposite diagonal direction. As a result, the respective films are stacked on top of one another by alternately applying stress in an “X”-shaped manner. In this manner, the WV polarizer 10 has the plural films bonded on top of one another by applying stress in the opposite diagonal directions. Here, since the films are vulnerable to heat and humidity, the films undergo stress caused by difference in expansion and contraction therebetween when exposed to the external heat and humidity for a long time period, so that weakened regions are formed near upper, lower, right and left edges of the polarizer 10. As a result, the WV polarizer suffers from light leakage when light is transmitted through these weakened regions exceeding a predetermined light transmission condition, which is referred to as an hourglass phenomenon.
FIG. 2 is a graph depicting brightness according to locations on a liquid crystal panel 50 in a normal operation, and FIG. 3 shows the liquid crystal panel 50 in the normal operation. As can be seen from these drawings, the overall region of the liquid crystal panel 50 exhibits substantially the same brightness in the normal operation. Namely, assuming the brightness at the center of the liquid crystal panel 50 is 100, the brightness at edges thereof is also 100.
FIG. 4 is a graph depicting brightness according to locations on the liquid crystal panel 50 upon occurrence of the hourglass phenomenon, and FIG. 5 shows the liquid crystal panel 50 upon occurrence of the hourglass phenomenon.
As described above, the weakened regions are mainly formed near upper, lower, right and left edges of the WV polarizer 10, and especially, the weakened regions are more easily formed near the right and left edges than near the upper and lower edges of the WV polarizer. Since the weakened regions allow a great quantity of light exceeding a predetermined condition to be transmitted therethrough, the hourglass phenomenon occurs at regions of a display screen corresponding to the weakened regions. Although the hourglass phenomenon is not clearly exhibited in an input of a white screen pattern, it becomes clear in an input of a dark screen pattern. Based on repetitious tests of an applicant of this invention, the WV polarizer had an approximately 20˜40% higher light transmittance at the weakened regions near the edges of the polarizer compared with the center of the WV polarizer. That is to say, assuming the brightness at the center of the liquid crystal panel 50 is 100, the brightness at the edges thereof is about 120˜140.
On the other hand, variation in physical properties of the WV polarizer caused by heat and humidity is attributed to inherent properties of the polarizer and is difficult to overcome only by improvement of a cell production process.